Process for boriding steel

ABSTRACT

STEEL IS BORIDED BY EMPLOYING A COMPOSITION INCLUDING A BORDING AGENT, AN ACTIVATOR, A DILUENT AND FREE CARBON. THE USE OF THE FREE CARBON PRODUCES PORE FREE BORIDE LAYERS OF NORMAL THICKNESS.

United States Patent 3,806,374 PROCESS FOR BORIDING STEEL HaraldKrzyminski, Konigstein, Germany, assignor to Deutsche GoldundSilber-Scheideanstalt vormals Roessler, Frankfurt, Germany No Drawing.Filed Mar. 13, 1972, Ser. No. 234,397 Claims priority, applicationGermany, May 27, 1971, 21 26 379.6 Int. Cl. C23f 7/00 US. Cl. 148-6 12Claims ABSTRACT OF THE DISCLOSURE Steel is borided by employing acomposition including a boriding agent, an activator, a diluent and freecarbon. The use of the free carbon produces pore free boride layers ofnormal thickness.

Several processes of boriding are known for the production of hardsurface layers on steels and other metals. The process differsessentially according to the type of boron imparting agent.

The treatment of the metals in boron imparting salt melts is knownaccording to two variants. Variant 1 takes place without electrolysis.In Variant 2 the deposition of boron is assisted electrolyticallywherein the part to be borided is the cathode while the crucible wall ora special electrode is used as the anode. The anode can also consist ofa boron containing material.

Besides there have been tried experiments to boride steel by highfrequency heating in aqueous boron containing solutions. For thispurpose there were preferably used aqueous solutions of borax or boricanhydride.

Further experiments have extended to the addition of gaseous boridingagents. For this purpose there are used as boron supplying gases boronhalides, diborane and various organic boron compounds. A special placebetween the liquid and gaseous boriding processes is occupied by theso-called drop gas boriding. In this process liquid boriding agents, forexample boron tribromide, boron trichloride and triethyl boron are addeddropwise into the preheated oven chamber and vaporized.

Finally there have been tried experiments with boriding pastes in whicha paste of a boron containing agent, a fluid agent and a binding agentare applied to the workpiece. The named processes until now have onlybeen tried and employed on a laboratory scale. A practical use has notbeen attained with any of the processes.

Attempts to employ the boriding in large scale have only led to successto a certain extent with powdered boriding agent. In the treatment theboriding with powders corresponds to the long known powder carburizing.The part to be treated is embedded with the boriding powder in boxes andsubjected to an annealing treatment for several hours in the temperaturerange of 800 to 1100 C. The boxes normally consist of steel plate andare closed with a cover of the same material. The boriding powderincludes the boron containing material and in many cases a diluent andactivating addition. As boron containing material there can be usedamorphous boron, crystalline boron, ferroboron, boron carbide and boraxas well as mixtures of these materials. As activating additives therecan be added the following compounds: potassium chloride, sodiumchloride, ammonium chloride, calcium chloride and barium chloride aswell as various fluorides, especially barium fluoride and magnesiumfluoride. Thus ammonium alkali metal and alkaline earth metal chloridesand fluorides can-be used as activating agents.

Very hard layers are produced on the parts by the treatment. In the caseof iron work pieces these consist of the compounds FeB and Fe B. Theboron rich compound FeB is formed only if the boriding activity of theagent is sutliciently strong. Since the FeB phase is only insig-3,806,374 Patented Apr. 23, 1974 nificantly harder than the Fe B phasebut is essentially more brittle, one is frequently content with a weakerboriding activity and strives for a boriding layer which is constructedonly of the compound Fe B. These layers possess a very high resistanceto wear which is why the processes today are employed in the industry toan increasing extent for various high load prefabricated parts andtools.

A serious deficiency of the boriding powders is that the outer zones ofboriding layers produced with them are permeated more or less stronglywith pores and oxidized inclusions. Inclusions are due to the fact thatthe powder structure contains oxygen or oxygen containing gases whichhave an oxidizing effect on the prefabricated parts. 0n the other handexperiments have shown that the presence of oxygen or oxygen containinggases is indispensable for the production of thicker layers. If thetreatment takes place in completely sealed boxes or in a vacuum, thestrength of the layer is reduced by about one half.

It has now been found unexpectedly that these deficiencies, namelyeither the formation of layers with oxide inclusions or the formation ofthinner layers, can be eliminated if the product added for boridingcontains free carbon in addition to the boron supplying substance, asfor example boron carbide. If there is used this type of mixture thelayers formed are pore free but still have the same thickness as thepreviously known borided layers, e.g. 220 to 240 am after a treatingtime of 5 hours at 900 0* Likewise their structure is the equal to thatwhich can be produced by the customary boriding powders, which meansthat the improvement of the properties is not reduced by the diffusionof carbon.

As boron containing materials there can be used amorphous boron,crystalline boron, ferroboron, boron carbide, borax, boric anhydride andmixtures of these materials in any proportions. These are mixed as iscustomary in the art with activating materials. Thus there can be usedthe activating materials set forth previously, e.g. ammonium, alkalimetal and alkaline earth metal halides, e.g. ammonium chloride,potassium chloride, sodium chloride, calcium chloride, barium chloride,potassium fluoride, barium fluoride, magnesium fluoride, sodium bromide,sodium fluoride and calcium bromide as well as mixtures thereof.

The composition can also contain the customary diluents such asgraphite, silicon carbide, aluminium oxide, and other stable compounds.

The novelty of the invention is based primarily upon the inclusion inthe mixture of free carbon, for example in the form of finely groundcharcoal or carbon black, or activated carbon. It has been found thatthe action of the mixture is best if the content of free carbon isbetween 2 and 40%, preferably between 5 and 15%. The amount ofactivating agent is usually between 2 and 10%, preferably 44% The amountof boron containing material is usually at least 10% of the compositionand can be as much as 96%. When a diluent is employed it us used in anamount of '5 to 88%.

Unless otherwise indicated all parts and percentages are by weight.

If the amount of free carbon is less than 2% there is no guarantee ofthe formation of pore free layers while, if the amount of free carbon isabove 40%, then the boriding activity is noticeably diminished.

The invention is primarily adapted to the boriding of steel but can alsobe used to boride other metals such as iron powder material parts, allkinds of cast iron, nickel and molybdenum. The boriding can beaccomplished at conventional boriding temperatures, e.g. temperatures ofto 1100 C.

*am 1/1000 mm.

EXAMPLE 1 (COMPARISON) A sample of steel Ck 15, normalized, was treatedfor hours at 900 C. in a boriding powder composed of 74% boron carbide(B C), 16% borax and potassium fluoride.

Then it was determined by metallographic examination that there wasformed a boride layer having a thickness of 220 to 240 millimicrons. Theouter part of this layer, however, was strongly porous and containedcoarse oxidic inclusions up to a depth of about 60 millimicrons.

EMMPLE 2 Another experiment was carried out with a mixture of 66% boroncarbide, 16% borax, 10% potassium fluoride and 8.% carbon, the latter inthe form of finely ground charcoal. Again a sample of the steel Ck wasborided for 5 hours at 900 C.

The metallographically ascertained boride layer had a thickness of 220to 250 millimicrons. The entire layer was pore free.

EXAMPLE 3 A further experiment was carried out with a mixture of 54%boron carbide, 16% borax, 10% potassium fluoride and carbon in the formof finely ground charcoal.

The metallographically ascertained thickness of the boride layerimparted to a sample of steel Ck 15 by boriding at 900 C. for 5 hourswas between 190 and 220 millimicrons. This layer also was pore free.

EXAMPLE 4 A further experiment was carried out with a mixture of 34%boron carbide, 16% borax, 10% potassium fluoride and 40% carbon black.

A sample of steel Ck 15 was borided for 5 hours at 900 C. in thiscomposition. The thickness of the boride layer as determinedmetallographically was then 150 to 170 millimicrons. The boride layerwas free of pores.

EXAMPLE 5 EXAMPLE 6- In this experiment a mixture of 20 boron carbide,10% sodium fluoride, 5% carbon in the form of finely ground charcoal and65% graphite was used.

Samples of nickel and molybdenum were borided for 5 hours at 900 C. inthis composition. The boride layer on the nickel piece had a thicknessof 120 millimicrons and consisted of the two compounds Ni B and Ni B.

The layer on the molybdenum sample was only 15 to 25 millimicrons thickand consisted of the compound MOBZ.

The layers were in all cases completely free of pores.

What is claimed is:

1. A process for the production of a pore free boride layer on a metalselected from the group consisting of iron, steel, nickel and aluminum,consisting essentially of placing an article made of said metal in aboriding powder composition containing a boron imparting agent selectedfrom the group consisting of amorphous boron, crystalline boron,ferroboron, boron carbide and borax, an activator and finely powderedactivated carbon in an amount of 2 to 40% of said composition andheating to 800 to 1100 C.

2. A process according to claim 1 wherein the metal is steel.

3. A process according to claim 2 wherein the activator is selected fromthe group consisting of ammonium halides, al kalimetal halides andalkaline earth metal halides.

4. A process according to claim 3 wherein the activator is used in anamount of 2 to 10% and is selected from the group consisting ofpotassium chloride, sodium chloride, ammonium chloride, calciumchloride, barium fluoride, magnesium fluoride, potassium fluoride andsodium fluoride.

5. A process according to claim 4 wherein the amount of activator is 4%.

6. A process according to claim 2 wherein the amount of free carbon is 5to 15%.

7. A process according to claim 2 wherein the composition consistsessentially of boron imparting agent, 2 to 40% of free carbon and 2 to10% of activator.

8. A process according to claim 7 wherein the boron imparting agent isselected from the group consisting of amorphous boron, crystallineboron, ferroboron, boron carbide and borax and the activator is selectedfrom the group consisting of ammonium, alkali and alkaline earthchlorides and fluorides.

9. A process according to claim 8 wherein the boron imparting agent is amixture of boron carbide and borax.

10. A process according to claim 1, wherein the heating is at 900 C.

11. A process according to claim 1, wherein the activated carbon isground charcoal.

12. A process according to claim 1, wherein the activated carbon iscarbon black.

References Cited UNITED STATES PATENTS 1,472,851 11/1923 Miyaguchi 14861,990,277 2/ 1935 Feussner et al. 1486 X 3,622,374 11/1971 Pike 117Dig.10 3,673,005 6/1972 Kunst 1486 RALPH S. KENDALL, Primary Examiner US.Cl. X.R.

1486.35; 117Dig. 10

